In a system wherein a propagating laser beam is employed for isotope separation or particularly uranium enrichment as described in U.S. Pat. No. 3,772,519 or U.S. Pat. No. 3,939,354, it is contemplated that a beam of radiation consisting of several colors of laser light will be propagated over distances of several hundred meters or more through ionization and separation chambers of the type illustrated in the above patents incorporated herein by reference. In passing through these chambers, typically containing an environment of uranium vapor, the propagating radiation beam will typically encounter a varying index of refraction. In particular, these variations may be such as to produce shifts in beam angle or actual beam displacements or a combination of these. While such shifts may appear slight within the dimensions of an individual chamber, over a long path of propagation of the radiation beam they can increase to produce an unacceptable misalignment in the beam. In addition, the propagating beam will experience a divergence, inherent in any laser beam which may or may not be augmented by its passage through a succession of ionization and separation chambers. While weak refractive elements could be introduced into the laser beam path to correct for the divergence periodically, such elements are likely to absorb unacceptably high amounts of radiation in high power laser beams resulting in not only a loss in efficiency but the generation of thermal stresses within the refractive element that could result in its failure. In addition, such a refractive element would not permit general readjustment of spacial and angular alignment of the beam.